Some of users having portable computers often have their computers operated with power supplied from a rechargeable battery. As a result, the rechargeable battery may undergo a cycle of charging and discharging per day, and the number of such cycles may reach up to 300 or more in one year.
As well known, the number of cycles of charging and discharging for a rechargeable battery is limited. When the number exceeds a threshold, for example, 300, the usable capacity of the battery will decrease significantly, for example, to 70% or lower of the original capacity of the battery.
As the usable capacity of a rechargeable battery decreases, the battery cannot continuously supply power for a time period as long as required by the user. In the worst case, the battery cannot work, and must be replaced with a new one.
The inventors have found that the conventional scheme of charging a rechargeable battery with a constant voltage or current is one of factors leading to a shortened lifetime of the battery.